1. Field of the Invention
The present invention relates to an over-the-highway vehicle and more particularly to an over-the-highway vehicle with a hydraulically driven power dolly.
2. Prior Art
Over-the-highway tractor-trailer rigs including a pair of trailers towed in tandem one behind the other have gained increasing acceptance as more and more states have legalized their use on turnpikes and highways of the interstate system. In fact, triples, that is rigs with three trailers, are now being used in some areas. In operational form, the tandem rig ordinarily comprises a relatively short tractor carrying a fifth wheel which couples with the forward trailer, and a dolly towed by the forward trailer and carrying another fifth wheel which couples with the rearward trailer.
In order to increase the traction and load hauling capacity of such tandem trailer combinations, proposals have been made for powering the dolly. One such proposal has been to provide the dolly with a power unit entirely separate from the tractor-carried power unit. Another approach has been to provide the dolly with a drivable axle, and couple the axle by a mechanical drive linkage to the tractor power unit.
One disclosure of the separate dolly engine approach is a paper by Norman B. Chew entitled "The Power Dolly Approach to Highway Transportation" submitted in October, 1965 at the Society of Automotive Engineers Combined Powerplant and Transportation Meeting in Cleveland, Ohio. One problem encountered with such separate dolly engine designs is that of controlling the dolly engine. Unless the power output of the dolly engine is regulated in accordance with the power output of the tractor engine, braking and jackknife problems may be created. Other problems arise in conjunction with the cost of providing and operating an entirely separate and distinct power unit.
The tractor-driven power dolly approach is disclosed in the referenced application. A drivable axle on the dolly is coupled by means of a flexible drive shaft to the tractor engine. While such an approach has the advantage of controlling the power output of the dolly in accordance with the output of a single prime mover, namely the tractor engine, it poses problems in conjunction with the delivery of power from the prime mover along the forward trailer to the dolly. Flexible mechanical drive couplings extending the entire length of a trailer are expensive and difficult to store. The positioning and connection of such couplings each time a trailer is added or dropped off is time consuming and costly.